The present invention relates to time interval measurements, and more particularly to an automatic period and frequency measurement feature for an analog oscilloscope having dual delay measurement capability that allows measurements to be made using a single sweep signal with a delayed trigger to detect edges.
An oscilloscope often is used to measure a time interval between events in an input signal. For example the period of a sine wave signal may be determined by measuring the interval between positive zero crossings of the signal, and the frequency determined from the inverse of the period. Traditionally such measurements are made using a display of the input signal on an oscilloscope screen. An operator determines the interval between the two events on the oscilloscope screen in units of "horizontal divisions" of a rectangular grid system superimposed over the input signal display. The operator multiplies the units by the sweep rate of the oscilloscope expressed in units of time/division to obtain the time interval. This crude measurement technique is slow and inaccurate.
To improve the accuracy of time interval measurements counter timers have been incorporated into oscilloscopes. The operator sets two cursors at the event points between which the time interval measurement is desired. The cursors determine trigger points on the horizontal sweep signal that initiate a counter/timer at the first cursor that counts the output pulses from a stable oscillator. The counter/timer is stopped at the second cursor, and the count is converted into the time interval by multiplying the count by the period of the oscillator output. Although this is an improvement in accuracy and speed, this technique requires the expense of the additional counter/timer hardware and the accuracy of positioning of the cursors by the operator.
A recent technique that does not use counter/timer hardware is disclosed in allowed co-pending U.S. patent application No. 07/213,645 filed June 30, 1988 by Gordon Wray Shank entitled "Time Interval Measurement System for Analog Oscilloscope." This technique uses both A and B ramp signals, with the A ramp signal triggering on a first event and the B ramp signal triggering on a second event. Initially both ramp signals trigger on the first event to set a first reference level, then the B ramp signal triggers on the second event and a second reference level is adjusted in an iterative procedure until both reference levels result in an indicator for both ramps occurring at the same time. The difference in levels between the two reference levels is converted into a time value for the interval between the two events. To obtain the most precise time interval measurements, both ramp signals may be characterized during calibration in the manner disclosed in co-pending U.S. patent application No. 06/902,363 filed Aug. 29, 1986 by Douglas Clifford Stevens and Henry Gregory Fox entitled "Sweep Generator Error Characterization" to compensate for sweep offset and non-linearity.
What is desired is an automatic period and frequency measurement feature for an analog oscilloscope that provides accurate time interval measurements that is simpler than previous techniques.